Resolution
By default, PHP limits uploaded files to 2 MB, and this limit applies to attachments on messages sent using Horde IMP. To change this value, the following parameter in php.ini needs editing: %plesk_vhosts%webmail\hordeupload_max_filesize = number of bytes
post_max_size = number of bytes + size of form data, e.g. message textmax_execution_time = 30 ;may need to be higher
memory_limit = -1 ;removes the memory limit for phpKeywords: horde maximum attachment size